1. Field of the Invention
The invention relates to a spindle unit and a tool clamping device for numerically controlled (NC) machine tool comprising a main spindle provided at its end with a means for clamping rotary tools.
2. DESCRIPTION OF PRIOR ART
It has been common practice for the spindle unit of machine tools to preferably employ large-sized and tough bearings in order to provide higher rigidity to a main spindle, and small-sized bearings are effective for suppressing heat generation at high rotational speeds of the main spindle. According to such common practice, the highly rigid spindles can be used only in case of lower rotational speeds, while the other spindles, designed for higher rotational speeds, inevitably are of lower rigidity. In other words, the prior art has taught that high rotational speeds and satisfactory high rigidity cannot be given to the main spindle.
A prior art spindle as shown in FIG. 7 comprising a main spindle 32 provided at its end with a means for clamping rotary tools and is formed to be a cylinder of hollow shaft rotatably supported by a spindle head 31 through bearings 33. Fitted in an axially extending hole of the cylinder is a tool-clamping device 37 as a part of the means for clamping the rotary tools. A tapered portion 34 is formed at a front end of the main spindle so as to tightly hold a rotary tool. In this example, a grind stone 36 is fixed to a tool holder 35 whose tapered portion 35a firmly engages with the tapered portion 34 of the main spindle. The tool holder 35 is pulled strongly and inwardly of the main spindle 32.
In such a prior art apparatus, an outer diameter of the main spindle 32 depends upon a size of the tapered portion 35a of the tool holder used. Therefore, the spindle 32 should, in general, have a large diameter such that it is difficult to drive the spindle to rotate at a high speed with a reduced size of the bearings. If such a large main spindle 32 is forcibly rotated at a high speed, then the bearings generate an increased amount of heat, thereby not only serious affecting the manufacturing precision but also giving rise to a burning of an jamming between the bearings and the spindle. Furthermore, the tool clamping device 37 and the axially extending hole of the main spindle 32 will, in such an event, unbalance the rotation thereof and cause a rocking rotation. As a result, undesirable vibration and noise from the apparatus are produced.
In addition, the main spindle 32 in the prior art apparatus is manufactured through two or more separate steps in order to form an inner hollow space, the taper portion, and other portions thereof. Consequently, the spindle is likely to loose its rotational balance. In order to avoid such a problem by improving the precision of the spindle, a manufacturing technique at an extraordinary high level of precision will be required to produce the main spindle.
It will thus be apparent that the prior art spindle units have indeed been incapable of precisely machining high precision mechanical parts.
However, there are increasing demands for high precision in high-speed grinding and high-speed cutting.